1. Field of the invention
The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery adapted to uniformly distribute stress applied to the electrode assembly when the battery is subject to a physical external force to prevent the battery from short-circuiting.
2. Description of the Related Art
In general, a secondary battery refers to a battery adapted to be charged and discharged, in contrast to a primary battery which is not chargeable, and is widely used in the cutting-edge electronic appliance field including cellular phones, laptop computers, and camcorders. Particularly, a lithium secondary battery has an operating voltage of at least 3.6V, which is three times larger than that of a nickel-cadmium battery or a nickel-hydrogen battery frequently used as the power source of a portable electronic appliance. The lithium secondary battery also has a high energy density per unit weight and, for these reasons, has rapidly prevailed in the industry.
Lithium secondary batteries generally use lithium-based oxide as the positive electrode active material and carbon material as the negative electrode active material. Electrolyte type lithium secondary batteries are generally classified into lithium ion batteries using a liquid electrolyte and lithium polymer batteries using a polymer electrolyte. Lithium secondary batteries are manufactured in various shapes including a cylinder, a square, and a pouch.
FIG. 1 is an exploded perspective view briefly showing a conventional can-type lithium secondary battery. A lithium secondary battery is formed by placing an electrode assembly 12 composed of a first electrode 13, a second electrode 15, and a separator 14 into a can 10 together with an electrolyte and sealing the top of the can 10 with a cap assembly 70.
The cap assembly 70 includes a cap plate 71, an insulation plate 72, a terminal plate 73, and an electrode terminal 74. The cap assembly 70 is coupled to the top opening of the can 10 and seals it. An insulation case 79 is installed in the upper portion of the electrode assembly 12 in order to prevent the electrode assembly 12 from contacting the cap assembly 70 and the can 10.
The cap plate 71 is a metal plate having a size and a shape corresponding to those of the top opening of the can 10. The cap plate 71 has a terminal through-hole formed at the center thereof with a predetermined size, into which the electrode terminal 74 is inserted. When the electrode terminal 74 is inserted into the terminal through-hole, a gasket tube 75 is coupled to the outer surface of the electrode terminal 74 and is inserted together for insulation between the electrode terminal 74 and the cap plate 71. The cap plate 71 has an electrolyte injection hole 76 formed on a side thereof with a predetermined size. After the cap assembly 70 is assembled to the top opening of the can 10, an electrolyte is injected through the electrolyte injection hole 76, which is then sealed by a plug 77.
The electrode terminal 74 is connected to the second electrode tab 17 of the second electrode 15 or to the first electrode tab 16 of the first electrode 13 and acts as a second or first electrode terminal. The remaining electrode not connected to the electrode terminal 74 is typically connected to can 10. Insulation tapes 18 are wound around the portions through which the first and second electrode tabs 16 and 17 are drawn from the electrode assembly 12, respectively, to avoid short-circuiting between the electrodes 13 and 15. The first or second electrode acts as a positive or negative electrode.
In the can-type lithium secondary battery, the insulation case 79 is inserted between the cap assembly 70 and the electrode assembly 12 to fix the position of the electrode assembly 12, as well as that of the electrode tabs 16 and 17, and is made of polypropylene material. The insulation case made of polypropylene material has a disadvantage of increasing the thickness of the battery, but the shape of the battery can vary with the manufacturer. Furthermore, the polypropylene material has weak rigidity and, when the electrode assembly is shifted toward the cap assembly during a drop test and the like, it deforms and causes local stress to be applied to the top of the electrode assembly. This results in short-circuiting. Although every manufacturer is trying to avoid short-circuiting resulting from such a physical external force by modifying the shape of the insulation case, the danger of short-circuiting still exists due to the rigidity of the polypropylene material itself.